Btw, I disagree with the notion that the aliens would just kill everyone instantly.
Historical precedent certainly suggests that 'Alien' invasions usually result in violence, and in the eventual subjugation of the technologically inferior side, but that usually takes a while, and it only very rarely leads to extinction. Cheap labour & reservations are much more likely outcomes.
I really, really don't think there'd be an asteroid bombardement, either. If Earth gets invaded, it's a reasonable assumption that the invaders want the environment reasonably intact.
Use relatively clean nukes or the equivalent thereof to reduce Earth' population to a reasonable level? Sure. Eliminate potentially troublesome parts of the population - most people with college degrees? Quite possible.
But complete extinction seems unlikely. It's just not what usually happens in first contact and invasion scenarios. It's not impossible, but like likelyhood is pretty low.
